The billing, custody transfer and usage of methane-rich gases, such as natural gas, fuel gas and bio-gas, is heavily dependent upon the energy content of the gas itself—i.e the amount of energy generated upon its combustion.
Energy, often measured in British thermal units (btu), is thus a critical measurement for gas suppliers, transporters and users alike. In addition to this, a parameter associated with the energy generated by the combustion of a gas (called calorific value or CV) is that of Wobbe Index (WI). This is an important parameter that indicates the ease with which a gas will bum.
The American Gas Association (AGA), in their Report No. 5, proposed a formula for the calculation of CV which has been broadly accepted and adopted. This formula is:CV=0.02035+(0.00197−[0.000329×Mco2+0.000217×Mn2])/SGwhere Mco2=% CO2 content; Mn2=% N2 content and SG is specific gravity.
For methane rich gases, there are two types of instrumentation that are commonly used to calculate/measure CV or WI—Gas Chromatographs (GC's) and Wobbe Index (WI) meters. GCs provide a relatively slow measurement technique in which the gas is separated into its constituent components and then the gas parameters are calculated by knowing the properties of the individual gases in the mixture. Wobbe Index meters either burn the gas and measure the energy released; or calculate CV or WI through one of a number of non-combustion techniques. For the non-combustion Wobbe Index meters, the major problem encountered during the measurement is that of understanding and accounting for the percentage of inert gases in the methane-rich mixture. Inert gases drastically change the energy content generated by the overall mixture, and thus need to be measured accurately.
GCs are widely used in the gas measurement industry as they are well established and are based on a known measurement method. Whilst they do output the full gas composition, and can calculate gas properties of interest to the user—such as density, base density, CV, WI, specific gravity (SG) etc—they have a number of significant limitations which include:                1. GCs are expensive to buy and have a number of moving parts which can give rise to significant servicing and calibration overhead costs;        2. Calibration gases that mimic the gas to be measured must be generated and this inevitably involves further expense;        3. Calibrations must be repeated typically every 6 to 8 weeks;        4. Skilled, trained operators are required to keep GCs functional & useful; and        5. GCs have a slow response time. Typically an answer is provided every 7 minutes however customers are requiring updates every 5 to 10 secs.        
Given this situation, if an alternative method can be found to calculate CV, WI, density, base density, SG etc, then significant opportunities exist in natural gas and fuel gas applications where fast gas energy measurement is required.
It is an object of the invention to provide a method which will go at least some way to addressing the limitations set forth above; or which will at least provide a novel and useful alternative.